A. Field of the Invention
This invention relates to the technology presently known as Internet telephony (sometimes referred to in the art as IP Telephony, where IP stands for Internet Protocol), in which signals from a caller station, such as a telephone, fax machine, modem, or video device, are transmitted, in the form of digital data packets, across a digital data network to a callee station. More particularly, this invention relates to a system and a method for Internet telephony that uses network address translation to translate the source and destination addresses of the digital data packets as they are transmitted from the address space of one network to the address space of another network.
B. Description of Related Art
Internet telephony is becoming an increasingly attractive alternative to the conventional public switched telephone network (PSTN) for long distance telephone calls because of its much lower cost and enhanced flexibility. To make a long distance telephone call using Internet telephony, a caller typically need only make a local telephone call to a local Internet telephony service provider. The Internet service provider than routes the telephone call to the Internet, often by using a gateway or terminal functioning in accordance with the H.323 and H.225 ITU-T standards. The H.323 and H.225 ITU-T standards are incorporated herein by reference. Another protocol that has been proposed is the “Session Initiation Protocol” (SIP). The most recent version of SIP is described in the Network Working Group's Request for Comments 2543 (March 1999), which is incorporated herein by reference.
Once the telephone call is routed to the Internet, its transmission is, for all practical purposes, free, regardless of its destination. Accordingly, in contrast to long distance service on the PSTN, which may charge users 10 to 30 cents per minute for domestic calls and considerably more for international calls, the cost of a long distance call using Internet telephony is potentially the cost of two local telephone calls (one at the calling end and one at the receiving end).
One problem with this conventional implementation of Internet telephony, however, is that it is not as user-friendly as the PSTN. Specifically, when a user desires to route a telephone call using conventional Internet telephony, the user typically must call a special telephone number, i.e., the telephone number of the Internet telephony service provider, and then dial the telephone number of the desired destination. In contrast, to use the PSTN, the user need only dial the telephone number of the desired destination.
One possible solution to this problem would be to bypass the PSTN entirely and provide telephones with direct Internet access. However, providing an Internet address for each telephone that uses Internet telephony may not be practical because of the increasingly limited number of available Internet addresses.
Finally, conventional Internet telephony has problems with privacy and security. The privacy concern relates to the extent to which the caller and callee can remain anonymous. The security concern relates to how easily an unauthorized party can gain access to the system, such as to make long distance calls without paying for them.